This invention relates in general to the use of imploding liners to achieve high energy densities and, more particularly, to apparatus for providing repetitive, stabilized implosions of liquid liners.
The use of imploding liners to achieve high energy densities from less extreme energy densities is well known. In the typical imploding liner compression system, a payload of relatively low energy density is confined in the bore of a cylindrical liner which is then caused to implode radially. During the implosion, the kinetic energy of the liner is converted by adiabatic compression to the internal energy of the confined payload which it surrounds.
The basic problem with earlier implosion systems was that operation resulted in the destruction of the apparatus, at least locally, and thus they were not well suited for repetitive applications. Two aspects of the previous implosion system operation introduced difficulties: (1) the use of explosives to impart high kinetic energy to the liner; and (2) the uncontrolled dynamics of the liner material before, during, and after peak compression. The former condition is largely historical, but is related to the need for high pressures at the outside surface of the liner to obtain high inner-surface speeds if the change in inner-surface radius during implosion is not large (r.sub.initial /r.sub.final .ltorsim. 10). The use of explosive detonation-drive generally requires the destruction of the apparatus. This problem may be eliminated by using non-explosive driving systems such as capacitor banks or high-pressure gases.
Difficulties in controlling the liner dynamics derive largely from mechanical instabilities, such s Rayleigh-Taylor instability, that are associated with the motion of the liner. For example, a basic hydrodynamic instability occurs when the interface between two fluids of different mass density accelerates in the direction of the heavier fluid (Rayleigh-Taylor instability). Thus, when the liner accelerates inward, its rear surface can be disrupted; and when the inner surface is decelerated in compressing the low mass-density payload (plasma and/or magnetic field, for example) it also will be disrupted. The reexpansion of the liner after peak compression is also subject to instability as the liner is slowed by the external driving fluid (gas or magnetic field). All told then, an initially well-defined fluid shape (cylindrical shell) will return to its original position with gross distortions and localized regions of high kinetic-energy density. This disruption of the liner will result in damage to the apparatus and prevent repetitive operation.
Recently, however large radial-compression-ratio liner implosions have been demonstrated using non-destructive techniques. See P. J. Turchi and A. E. Robson, Proc. of Sixth Symposium on Engineering Problems of Fusion Research, San Diego, Cal., Nov. 18-21, 1975 IEEE Publication No. 75CH1097-5-NPS. p. 983. This approach introduced the concept of rotationally-stabilized liquid metal liners accelerated with radially-displaced free pistons as a means of controlling the liner dynamics. During acceleration, the liner was stabilized on its outer surface by the pistons and on the inner surface by the centripetal acceleration due to the rotation of the liner. The application of stiff, radially-displaced free-pistons to the outer surface prohibits the growth of high-frequency Rayleigh-Taylor instabilities, but does not, however, restrict the growth of lower-frequency modes. Such growth results in variations in free-piston positions and in a non-uniform, asymmetric distribution of fluid mass and momentum. Indeed, variations in piston position and low-mode-number asymmetries of liner mass distribution have been observed in experiments with rotating liners driven by a plurality of radially-displaced pistons. The combination of this basic hydrodynamic instability with other factors, such as variations in reaction to Coriolis forces on the radially-moving pistons in the rotating cylinder block, and reliability considerations of the statistically large piston numbers, indicates the need for synchronizing mechanisms to insure the uniformity of the liner implosion. With radially-moving pistons, a concatenation of individual mechanisms, such as gears, cams, tie-rods, valves, etc., would be needed to couple the piston motions. The number of moving parts in the system would increase as some multiple of the number of pistons.